1. Field of the Invention
The present invention relates to an enlarging lens system for photomechanical process.
2. Description of the Prior Art
Unlike general photographic lenses, lens systems for photomechanical process must be so designed as to uniformly correct various aberrations when objects at short distance are photographed since such lens systems are used for copying plane objects at short distance with high fidelity. Lens systems for photomechanical process must assure high image flatness and little distortion especially on large image planes. For satisfying these requirements, lens systems for photomechanical process must have long focal length and, therefore, it becomes rather difficult to design lens systems having large aperture ratios. Especially in case of enlarging lens systems, however, they should preferably have aperture ratios as large as possible since light intensity is attenuated so much during half-tone separation, masking, etc. in the photographing process that a long exposure time is required. Especially on a large image plane, image flatness is degraded and satisfactory images may not be obtained in circumferential areas.